This application relates to lamps, and particularly a ceramic metal halide (CMH) lamp, and improving the performance of CMH lamps by reducing heat loss associated with the lamp assembly. More particularly, the disclosure is related to controlling heat loss from the arc chamber to the legs with resultant increased efficiency of light radiation.
CMH lamps have become increasingly popular due to significant customer benefits. Traditionally, quartz arc tubes have been commonly used in metal halide arc discharge lamps. More recently, however, there has been a trend toward using CMH lamps that include a ceramic arc tube because of better color uniformity and stability, as well as increased lumens per watt (LPW) and color rendering (Ra) relative to traditional arc discharge lamps. These performance advantages of ceramic arc tubes are enabled by their higher temperature compared with quartz arc tubes and by a reduced rate of sodium loss.
In discharge lamps of this type, efficacy and lamp performance are adversely affected by the loss of energy through thermal conduction along the legs, or ends, of the arc tube. Commonly assigned U.S. Pat. No. 6,621,219 shows and describes one manner of limiting axial heat flux along the arc tube leg by designing the leg structure to have a reduced thermal conductivity, and the details of that disclosure are fully incorporated herein by reference. Particularly, reducing the molybdenum mandrel diameter effectively reduces the thermal conductivity of this component even when the diameter of the overwind wire is increased or when multiple overwinds are used. That is, the overwind provides a distinct reduction in thermal conduction along its length because of the helical conformation when compared to the mandrel portion.
There are three primary areas of thermal conductance in a direction generally parallel to the leg axis. A major thermal conductance is in the leg itself, or the polycrystalline alumina (PCA). The next largest thermal conductance is along the molybdenum mandrel. The third area of thermal conductance, and also the smallest, relates to the molybdenum overwind.
Miniaturization, improved color quality (Ra over 90) and energy efficiency are three major industry trends in the area of CMH technology development. As CMH lamps are made even smaller, it becomes more difficult to achieve a target seal glass temperature, a shorter overall arc tube length that fits inside an outer jacket, and simultaneously achieve targeted photometric performance, i.e., lumens, Ra, etc. Other parties have reduced the length of the leg or the length of the arc chamber, but provide a thicker leg. Consequently, performance of the resultant lamp is not as desired.
A need therefore exists to provide a CMH lamp that avoids an increase in seal glass temperature, has reduced thermal conductivity along the leg and thereby results in more energy distribution into the arc for light, and enables shorter leg length that eases fitting of the arc tube into an outer jacket and provides greater flexibility in arc tube body design to optimize performance.